A farmer walked through his field one cold winter morning. On the ground lay a snake, stiff and frozen with the cold. The farmer knew how deadly the snake could be, and yet he picked it up and put it in his bosom to warm it back to life. The snake soon revived, and when it had enough strength, bit the man who had been so kind to it. The bite was deadly and the farmer felt that he must die. As he drew his last breath, he said to those standing around, "Learn from my fate not to take pity on a scoundrel". 
There are some who never changes their nature, regardless of how good we behave with them. Always stay alert and maintain the distance from those who are there only thinking about their own benefits. 